This invention relates to dynamic interferometers used to determine the wavelength of an unknown signal or used to determine the position of a movable element.
U.S. Pat. No. 4,329,055, Schaefer et al., describes an interferometer using a rotating parallelpipedic transparent body. As shown in FIG. 1 (prior art), which is simplified rendering of FIG. 1 of the Schaefer patent, Schaefer discloses the use of a laser source 2 that sends an input beam to the beam splitter 4. The beam splitter 4 at point 4a splits the incoming light beam into two arm beams. The two arm beams travel through the rotating parallelpipedic transparent body 5 and are reflected off mirrors 6 and 7. The two arm beams then return to point 4a in the beam splitter 4 and are recombined into a interference signal beam that is sent to detector 8. The interference signal detected at the detector 8 will have an intensity that depends on the wavelength of the input beam and the path length difference between the arm beams. This path length difference is produced in the rotating parallelpipedic transparent body 5 and changes as the parallelpipedic transparent body 5 rotates.
Although only one input signal is shown in FIG. 1, Schaefer et al. discloses the use of both an input signal of a known wavelength which is split into reference arm beams and an input signal of an unknown wavelength which is split into unknown wavelength arm beams. The use of an input signal of a known wavelength increases the accuracy of the determination of the unknown wavelength. The input signal of the known wavelength and the input signal of the unknown wavelength are sent into the apparatus in parallel along different paths. Since the reference arm beams comprise different paths from the unknown wavelength arm beams, the reference arm beams and the unknown wavelength arm beams will have different contact points with the reflective surfaces of the parallelpipedic transparent body 5 and the interferometer will be sensitive to any lack of flatness of these reflective surfaces. Another disadvantage of the interferometer disclosed in Schaefer is that the refractive index of the rotating parallelpipedic transparent body 5 is wavelength dependent so that beams with different wavelengths refract at different angles inside the rotating bodies.
It is an objective of the present invention to provide a wavelength meter with improved accuracy.